The general aims are first, to detemine something of the relation between a light stimulus incident on a human eye and the visual signals generated by the rod and cone recepotrs which absorb the light and second, to elucidate the neural mechanisms that modify these signals as they are transmitted through the retina. In particular, psychophysical investigations of rod saturation are proposed, that should help elucidate (a) the receptoral origin, (b) the temporal properties, and (c) the spatial spreading of the signals involved.